The sensation of pain is subjective and unique to each individual because the experience of pain depends on both the input of noxious information (nociception) and the state of the body - an interaction between pain transmission and pain modulation. The transmission of nociceptive information through the spinal cord dorsal horn is subject to bidirectional control by brainstem pain-modulation systems. The output of the best-studied modulatory system is through the rostral ventromedial medulla (RVM), which exerts bidirectional modulatory effects mediated by two sets of physiologically identified neurons: the ON-cells and the OFF-cells. The RVM ON- and OFF-cells respectively facilitate or inhibit the passage of noxious information from the periphery through the dorsal horn. However, the pathway through which noxious inputs gain access to the RVM is unknown. My hypothesis is that the spinoparabrachial pathway, the major ascending output from superficial laminae of the dorsal horn, is the primary source of noxious inputs to the RVM ON- and OFF-cells. This hypothesis will be tested through completion of two Specific Aims. First, noxious inputs to RVM neurons will be studied using in-vivo single cell recordings and pharmacological inactivation of the parabrachial complex (Aim 1a). Employing retrograde tracing and immunohistochemical techniques, the anatomical connection between the parabrachial (PB) nociceptive neurons and the RVM neurons will be established (Aim 1b). Finally, the functional role of direct PB-to-RVM circuit will be explored through the use of optogenetic methods (Aim 2). There is growing evidence that failure or abnormality in pain modulating systems is an important factor in many chronic pain states. However, we do not yet know how nociceptive information reaches pain-modulating neurons in the brainstem. The proposed studies will address this gap in our knowledge, delineating the pathways through which pain transmission systems link to pain modulation systems. This information is critical if we are to develop better approaches to treating chronic pain.